1. Field of the Invention
The present invention relates to a burner, and more particularly, to a burner for heating systems.
2. Description of Prior Art
Various concepts are known from the prior art for the reduction of the noxious substances, such as NO.sub.x or CO, which arise during combustion. Since the NO.sub.x production is large at high combustion temperatures, one attempts, for example, to keep the flame temperature low. For this purpose a heating boiler has been proposed, for example in EP 0 256 322 B1, in which a fuel gas is burned at a temperature of less than 700.degree. C. through the use of a catalyst of the platinum group, whereby the creation of nitrogen oxides is prevented. However, such catalysts have only a relatively low working life and are, moreover, very costly. The essential disadvantage of catalytic combustion, however, lies in the fact that its flame temperature is too low, which does not permit any effective exploitation of the heat and thereby only allows the construction of a burner with a low power density.
In addition to this, there are burners which operate in accordance with the process of exhaust gas recirculation. Here a part of the exhaust gas is returned into the flame, whereby an optimized, pollution reduced combustion is achieved. A stable flame arises with the burner model "RotriX" of the Viessmann company through an intentional decay of the turbulent fuel/air mixture, which has been set into rotation. The exhaust gas recirculation rate can be further increased by a flameless oxidation at a free surface. According to the specialist paper by J. A. Wunning and J. G. Wunning: "Brenner fur die flammlose Oxidation mit geringer NO-Bildung auch bei hochster Luftvorwarmung" (Burner for the flameless oxidation with low NO-formation even with the highest air pre-heating), in GASWARME International, Vol. 41 (1992), No. 10, pages 438 to 444, the flameless oxidation is usable in burners with process temperatures over 850.degree. C. This process, however, involves high constructional cost and complexity because auxiliary burners are required, for example, for the heating up of the fuel/air mixture to ignition temperature.
A further concept is present in the form of the "Thermomax-Burner" of the company Ruhrgas AG, which is treated in the specialist paper by H. Berg and T. Jannemann "Entwicklung eines schadstoffarmen Vormischbrenners fur den Einsatz in Haushalts-Gaskesseln mit zylindrischer Brennkammer" (Development of a low-pollution pre-mixing burner for use in domestic gas boilers with a cylindrical combustion chamber), in GASWARME International, Vol. 38 (1989), No. 1, pages 28 to 34. The combustion takes place there in a flameless manner at the surface of a metallic, apertured sheet, which transmits the heat energy produced out of the reaction zone principally by radiation. The combustion temperature is kept to approximately 800.degree. C. through this giving off of heat, which in turn has the consequence of a reduction of the emission of pollution. Burners of this type of construction typically have a thermal surface loading of 300 kW/m.sup.2.
An increase of the thermal loading to approximately 3000 kW/m.sup.2 is achieved by a burner which is known from DE 43 22 109 A1. There, a part of the combustion chamber, in which a flame propagates, is completely filled with a porous material whose porosity changes along the flow direction of the fuel gas/air mixture in such a way that a critical Peclet number results at a boundary surface, or in a specific zone of the porous material, from which point on a flame can arise. With regard to the Peclet number, the following should be explained:
With a specific pore size of the porous material, the production of heat by chemical reactions in the flame and the dissipation of heat by the porous medium are equal so that beneath this pore size no flame can arise but above it a free ignition occurs.
This condition is described with the aid of the Peclet number, which recites the ratio of heat production to heat dissipation. In this way a critical Peclet number results for the flame propagation. A self-stabilizing flame within the supercritical zone results through the provision of a subcritical zone and a supercritical zone with respect to the Peclet number.
Through the arrangement set forth in DE 43 22 109 A1, the problem of the stability of a flame burning in a porous medium is solved under the side conditions of a low temperature and thus a low emission of pollution. Ceramic foams or bulk fillings of balls are proposed as porous material. These materials have, however, a relatively low porosity, whereby combustion space is wasted and the gas/air mixture is exposed to a higher flow resistance. Moreover, these materials restrict, as a result of their low optical permeability, the energy transport on the basis of the thermal transport mechanism of thermal radiation which dominates in the present temperature range. This leads to a situation--from a specific constructional size of a burner of this kind onwards--in which the heat produced cannot be dissipated sufficiently well outwardly from the inner region of the combustion space. The local overheating in the porous material brought about in this way leads to material damage by thermal strains and an increased output of pollutants.